This invention relates to a liquid gas heat exchanger for transmitting heat from a heated liquid to a cooler surrounding gaseous medium and to a method of manufacturing same. The invention finds particular utility in the manufacture of automobile and truck radiators. It can also be used in the manufacture of automotive air conditioning heaters, as well as industrial and commercial heat exchangers.
A typical vehicular radiator consists of an upper and lower header plate connected by an array of thin walled liquid conducting tubes, usually made of metals or plastics. The header plates have punched openings surrounded by upset collar portions which receive the ends of the tubes. The tube ends are joined to the collar portions in a liquid tight joint by means of various metal joining techniques, such as welding, brazing, soldering and plastic bonding. The top header plate is closed off to form an intake tank to receive the hot coolant from the engine power plant. The bottom header plate is closed off to form a discharge tank to receive the relatively cooled liquid and return it to the power plant cooling system. The tube array is bonded to extended heat transfer surfaces to dissipate the heat to ambient air.
Automobile radiators of the above basic design have been in use from an early period in the development of the automobile. It was soon discovered, however, that radiators failed in service much earlier than anticipated. Failure most frequently occurred at the joints between the tubes and the openings in the header plates. The factors causing failure at the joints are as follows: (1) stresses caused by thermal expansion and contraction of the tubes and header plates; (2) stresses caused by changes in pressure within the tubes; (3) fatigue failure caused by vibrations generated by the engine and road travel; and (4) internal corrosion of the joints by the cooling liquid and additives, as well as external corrosion caused by road salts and chemicals.
A significant effort in radiator design and development has been directed to improving the structural integrity of the tube to header joints. A number of patents have been issued dealing with this problem. U.S. Pat. No. 3,245,465, issued on Apr. 12, 1966, to Fred M. Young, shows a punched header plate to receive an array of oval-shaped liquid conducting tubes. Each punched opening is provided with an extended upset collar portion to increase the strength and contact surface area between the collar and tube. Molten solder is drawn in by capillary action to seal the tube to the collar. As an added feature, a small support plate may be mounted on the tube and bonded to the underside of the header for additional support.
U.S. Pat. No. 4,529,034, issued on Jul. 16, 1985, to Zalman P. Saperstein, shows a punched header plate provided with upset collars to receive the tube ends. The end portion of each tube end and collar are welded to each other on the wet side of the plate to form a complete peripheral weld. For additional support and to plug any cracks or pin holes in the welded joint, the space in the collar to tube joint on the air side of the weld is filled with solder to form a fillet.
U.S. Pat. No. 4,858,686, issued on Aug. 22, 1989, to Gerald C. Calleson, shows a header plate with circular punched openings and circular upset collars. In order to join conventional oval-shaped tubes to circular collars, the tube ends are expanded from oval to circular shape with an external diameter to fit within the collars with a clearance suitable for soldering, brazing or welding. The joint is sealed by soldering, brazing or welding, or any combination of the two. The joint is said to be of increased strength by virtue of the increased contact surface available for bonding.
Although the technology as exemplified by the patents described above has extended the service life of vehicular radiators, a demand has arisen in recent years for a level of quality and length of service not attainable by the prior art. Applicants have made a major improvement in joint design which will advance radiator reliability and length-of service far beyond the present state of the art.
This invention also relates to a novel method of manufacturing a radiator incorporating applicants' novel tube to header joint. In the prior art manufacturing process, each individual joint was welded by tracing a path just above and around the periphery of the joint with the arc generating tungsten tip. The welder is programmed to move from joint to joint, repeating the peripheral welding process each time, until a row of joints is completed. The process is again repeated for an adjacent row until all the rows are welded. It is also known to approximate a complete peripheral weld by making a number of linear passes down a row of joints. It can be seen that the prior art processes can be quite time consuming and expensive.
Applicants have devised a method wherein the welder traces a linear path down a medial portion of a row of joints in a single pass. The process is repeated for each row yielding a great savings in time and cost.